This invention relates to a data transmission circuit for use in transmitting a local transmission signal through a transmission line of a loop type and to a transmission system which comprises a plurality of the data transmission circuits connected through a loop-shaped transmission line.
In a conventional system of the type described, a plurality of data transmission circuits are connected to a loop-shaped transmission line at multipoints. Each data transmission circuit unidirectionally transmits a local transmission signal through the transmission line. When an order wire signal is transmitted as the local transmission signal from a selected or reference one of the data transmission circuits to all of the remaining data transmission circuits, through the loop-shaped transmission line, the order wire signal is returned back to the selected data transmission circuit. This is because no destination address is included in such a local transmission signal. In this event, the same local transmission signal is cyclically and repeatedly transmitted through the loop-shaped transmission line and is repeatedly received by the selected data transmission circuit in which such a local transmission signal is unnecessary. Such repeated reception and transmission of the same local transmission signal should be avoided in the transmission system and, otherwise, any local transmission signals can not be transmitted to the transmission line.